Platinum: the essentials

Ruthenium, rhodium, palladium, osmium, iridium, and platinum together make up a group of elements referred to as the platinum group metals (PGM).

Platinum is a beautiful silvery-white metal, when pure, and is malleable and ductile. It has a coefficient of expansion almost equal to that of soda-lime-silica glass, and is therefore used to make sealed electrodes in glass systems.

The metal does not oxidise in air. It is insoluble in hydrochloric and nitric acid, but dissolves when they are mixed as aqua regia, forming chloroplatinic acid (H2PtCl6), an important compound. It is corroded by halogens, cyanides, sulphur and alkalis. Hydrogen and oxygen gas mixtures explode in the presence of platinum wire.

Platinum: historical information

Platinum was discovered by Antonio de Ulloa in 1735 at South America. Origin of name: from the Spanish word "platina" meaning "silver".

The metal was used by pre-Columbian Indians but platinum was "rediscovered" in South America by Ulloa in 1735 and by Wood in 1741. In 1822 plenty of platinum was discovered in the Ural Mountains in Russia.

Platinum is one of the elements which has an alchemical symbol, shown below (right, {{floatR}}{{/floatR}} alchemy is an ancient pursuit concerned with, for instance, the transformation of other metals into gold).

Sometime prior to the autumn of 1803, the Englishman John Dalton was able to explain the results of some of his studies by assuming that matter is composed of atoms and that all samples of any given compound consist of the same combination of these atoms. Dalton also noted that in series of compounds, the ratios of the masses of the second element that combine with a given weight of the first element can be reduced to small whole numbers (the law of multiple proportions). This was further evidence for atoms. Dalton's theory of atoms was published by Thomas Thomson in the 3rd edition of his System of Chemistry in 1807 and in a paper about strontium oxalates published in the Philosophical Transactions. Dalton published these ideas himself in the following year in the New System of Chemical Philosophy. The symbol used by Dalton for platinum is shown below. [See History of Chemistry, Sir Edward Thorpe, volume 1, Watts & Co, London, 1914.]

Platinum is found as the free element, usually mixed with other metals including gold, nickel, copper, palladium, ruthenium, rhodium, iridium, and osmium in Columbia, the USA, Ontario (Canada) and the Urals (USSR). Platiniridium is a naturally occurring platinum/iridium alloy. Platinum is also found in ores such as sperrylite (platinum arsenide, PtAs2) and cooperite (platinum sulphide, PtS).

Isolation

Isolation: it would not normally be necessary to make a sample of platinum in the laboratory as the metal is available commercially. The industrial extraction of platinum is complex as the metal occurs in ores mixed with other metals such as palladium and gold. Sometimes extraction of the precious metals such as platinum and palladium is the main focus of a partiular industrial operation while in other cases it is a byproduct. The extraction is complex and only worthwhile since platinum is the basis of important catalysts in industry.

Preliminary treatment of the ore or base metal byproduct with aqua regia (a mixture of hydrochloric acid, HCl, and nitric acid, HNO3) gives a solution containing complexes of gold and palladium as well as H2PtCl6. The gold is removed from this solution as a precipitate by treatment with iron chloride (FeCl2). The platinum is precipitated out as impure (NH4)2PtCl6 on treatment with NH4Cl, leaving H2PdCl4 in solution. The (NH4)2PtCl6 is burned to leave an impure platinum sponge. This can be purified by redissolving in aqua regia, removal of rhodium and iridium impurities by treatment of the solution with sdoium bromate, and precipitation of pure (NH4)2PtCl6 by treatment with ammonium hydroxide, NH4OH. This yields platinum metal by burning.

- Platinum isotopes are used in the following fields. Pt-196 is used in experiments to test nuclear models. Both Pt-194 and Pt-196 have been used in research into dipole strength and models. Pt-195 is used for the production of the radioisotope Pt-195m which is used for cancer diagnosis and therapy. Pt-198 is used for the production of the radioisotope for Au-199 which is used in cancer therapy. Pt-194 is also used for the production of the medical radioisotopes Hg-195m.